The invention relates to systems which provide earth-based observers the capability to "see" over-the-horizon and, more particularly, to either a satellite or missile based system for collecting and communicating information about targets which are over the earth's horizon from the receiver of the information.
A fixed or large, slow-moving asset (such as a ship) is at a disadvantage when defending itself against a more mobile adversary because the adversary may relatively easily discover the ship's location. Moreover, the defense does not have targeting information until the adversary or its weapons come within range of the defensive sensing and targeting systems. Unfortunately for the defense, because of the high speed of modern weapons delivery systems, the time to respond is very compressed and places a strain on the defensive systems. Obviously, the more the defense can extend the range of its sensing and targeting systems, and thereby increase its time to respond, the lesser the advantage of the offensive adversary.
A number of systems such as airborne early-warning systems and relocatable over-the-horizon radar have been developed to provide long-range over-the-horizon surveillance. Those systems can provide detections with relatively coarse resolution, but they are limited in providing the kind of targeting information (such as the number and precise location of targets) required for the effective commitment of long-range defensive missiles.
Space-based sensor systems have been considered for surveillance missions, but their affordability is reduced when the additional requirements of targeting are imposed. Some of the reasons for the high costs of satellite systems are the long ranges at which the sensor must operate at orbital altitudes and the large number of satellites required by orbital dynamics in order to maintain surveillance and targeting capabilities over a single geographical area. Consequently, there is a need for a low-cost over-the-horizon targeting system.
A system for providing over-the-horizon targeting will, of necessity, be different from one providing over-the-horizon surveillance because of the different requirements of surveillance and targeting. Whereas both systems must provide a detection capability at long ranges from the defended asset, target information must be more timely and more accurate for a targeting system than for a surveillance system. Also, a surveillance system must be able to cover all areas of interest, while a targeting system can concentrate on smaller areas containing potential targets. A surveillance system must have continuous time coverage, whereas a targeting system only has to provide information when needed, e.g., during the course of an engagement. As with surveillance, a targeting system must be able to obtain information in all kinds of weather and to transmit that information to appropriate receiving stations in all kinds of electronic countermeasure environments. Finally, any system must be reasonably affordable.